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Bring xenon tank mass ratios to standard values


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The Story So Far:

Prior to KSP v1.0, mass ratios of tanks varied wildly. For some fuel types, there was a consistent progression across tank sizes, for others, there was not; and across fuel types, there was no consistency at all.

In KSP v1.0, Squad made the decision to standardize all the fuel tanks in KSP to a mass ratio of 9:1 - meaning one ton worth of tankage holds 8 tons worth of propellant.

All the fuel tanks...? No! A small holdout yet remains in the northern reaches of Gaul in the form of the xenon tanks. Their mass ratio still sits at 2.2727:1 - meaning one ton of tankage holds just 1.2727 tons of propellant. It's almost a fifty-fifty split between dry mass and xenon!

 

Why It Matters:

The rocket equation consists of two parts that are multiplied with each other. One is the effective exhaust velocity, which we know in the derivative form of specific impulse (Isp). The other is the vessel's mass ratio. Therefore, the mass ratio is at least as important as Isp for the performance of a rocket. You can argue that it is even more important, because while Isp only defines the rocket's dV, the mass ratio defines the rocket's dV in the same way as Isp does, and in addition to that, also defines the rocket's TWR. A better mass ratio gives you both more dV and shorter burns.

Another consequence of the mass ratio going into the rocket equation lies in the maximum dV any given rocket stage can achieve with a specific engine (more precisely, with that engine's Isp). Because even if you add the entire observable universe's mass in terms of fuel to a rocket that somehow weighs zero kg despite having an engine and other stuff, you could still not exceed the mass ratio of the tanks you are using. Therefore there's a hard limit of dV you can never exceed with any given engine, and it depends entirely on the tank's mass ratio - and the way it curves towards that limit with increasingly diminishing returns depends on it, too.

 

Why It Should Be Addressed:

In real life, the mass ratio of a fuel tank depends on a great many factors, including the physical properties of the propellant itself. So you could argument that it is realistic that different fuels have tanks with different mass ratios. However, for xenon specifically, real life tanks are not anywhere near that low.

In addition to that, as mentioned above, Squad made the conscious decision to remove all tankage mass ratio differences from the game (except xenon, which as it would appear was forgotten). And there's good reasons for doing this, reasons I've experienced myself multiple times while helping out with the mod Near Future Technologies. One pack in this suite of mods introduces new engines, which run on several different new fuels. I'm doing the balancing of those engines, and over time, I've tried various different approaches. Among them were approaches that had all the fuels at different tankage mass ratios. Although this offered increased freedom in assigning stats and part niches, it also made everything a whole lot more complicated to manage and balance properly. But that's not the main reason it was a bad idea.

No, the main reason is that none of the players noticed it. None whatsoever. Not a single person who hadn't followed the dev thread discussion ever admitted to being aware of these differences. And, I mean, why would they? Fuel tanks are just fuel tanks. When you build literally any other spacecraft in KSP, you plop down the payload, you plop down a number of fuel tanks, and then you sit down and start comparing engines for thrust and Isp. Engines, not tanks. So the only thing those players noticed was: some engines just had better stats than others, no matter how you turned it. And these players then came to the mod thread and complained that their favorite engine was underpowered, or that another one was really broken, and that our balancing was super bad. All in all, mucking with tank mass ratios turned out to be unnecessary complexity that didn't add any depth, only confusion.

Squad, I wager, knows this full well, for they have a forum full of active users who love to discuss engine balance while equipped with many different variants of half the data. And with the introduction of such things as the monoprop engine, and the move of the LV-N to be liquid fuel only, KSP moved rapidly towards a situation where engines could not be directly compared to each other anymore. So they made LF/Ox, LF-solo and Monoprop tanks all have the same mass ratios. Voila, engines could now be compared again!

Except, you know, the often-forgotten Dawn ion engine.

 

What The Actual Effects Are:

The Dawn has so wildly different stats from all other engines, both in thrust and Isp, that comparing it to other engines is fairly straightforward on the surface: the dV you get is simply just "higher", and the thrust you get is simply just "lower". It clearly gives the Dawn its niche. However, if you sat down and compared the actual stats, you'd quickly discover that this engine doesn't perform the way you think it does. Namely, it always significantly underperforms compared to what your on-paper math says it should do. And the more fuel you add, the worse it gets.

To illustrate, I've done some math on a vessel that is comprised of 10 tons of dry mass, including a Dawn engine. I've added enough fuel to hit several different propellant fraction targets (percentage of vessel mass that is fuel). At each of those targets, I determined how much more dV and TWR the vessel would get if its tank mass ratio was 9:1 instead of 2.2727:1... and then converted that into how much less "effective Isp" and "effective thrust" the Dawn gets compared to a hypothetical LF/Ox engine with the same stats. I also calculated the difference in total mass between the vessel with the LF/Ox tanks and the vessel with the xenon tanks. Spacecraft mass is important when designing your lifter, after all... and that difference is the extra mass you have to lift just because you're using xenon (in addition to being responsible for the drop in effective thrust).

Base Stats 4200 s 2.00 kN n/a
Propellant Fraction Effective Isp Effective Thrust Vessel Mass Difference
15% 3739 s 1.80 kN 1.38 tons
20% 3564 s 1.73 kN 2.09 tons
25% 3378 s 1.66 kN 3.04 tons
30% 3175 s 1.59 kN 4.36 tons
35% 2952 s 1.53 kN 6.29 tons
40% 2703 s 1.46 kN 9.44 tons
45% 2417 s 1.39 kN 15.45 tons
50% 2072 s 1.33 kN 31.50 tons

As you can see, the Dawn effectively drops to barely half the performance a player would infer from looking at its stats alone by the time the propellant fraction hits 50%. This is not the Dawn engine's fault. It's the xenon tank's fault.

 

What I Propose:

Simply put, adjust the dry masses of the xenon tanks to fall in line with all the other fuel tanks in KSP. I've even gone ahead and calculated the numbers - you just need to put them into the configs:

- PB-X50R: 0.03143 becomes 0.005
- PB-X150: 0.055 becomes 0.00875
- PB-X750: 0.4125 becomes 0.065625

Meanwhile, doing this obviously results in a massive buff to the Dawn engine. But consider the following: because of the poor xenon tanks currently in the game, the Dawn engine needs to have artificially inflated stats (in both thrust and Isp) in order to deliver the performance that it is designed to have in typical usage scenarios. As such, there should be no problem with adjusting those stats down once the tanks are no longer killing it.

Since the Dawn engine is a direct nod to the NSTAR ion thrusters on NASA's Dawn spacecraft, currently in orbit of Ceres, why not take the Isp directly from that? Instead of 4200s, it would now have 3120s. Looking at the table above, that falls pretty much straight in the middle of what you effectively get right now anyway, on average... a perfect fit. The thrust could be set at 1.5 kN, a nice round number that's also reasonably near the average effective performance today. 1.6 kN would also be valid, but from my gut, I'd choose the lower value. Xenon-fueled spacecraft are already getting another stealth buff in requiring less lifting power to put them into orbit.

 

In Closing:

From my amateur viewpoint, I consider this change to be simple, straightforward and requiring only little testing - it doesn't affect many game systems, and the required changes are so few and simple, a ModuleManager script could do it.

I'd love to hear everyone's thoughts and comments on it! :)

 

Edited by Streetwind
Typos, typos everywhere!
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Hm, if I don't read the technical manuals from orbital ATK wrong, their fuel tanks have ratios of ~4.4:1 (fuel to dry mass). So not close to 9:1, but definitely more then 2.2:1.

 

I think you have reasonable ideas to offset the gains from better tanks by lowering the values of the engine. 

 

But I don't know if there really is a problem to solve. You use Xenon for upper stages when they are small enough not to warrant using an LV-N, and if you use can use an LV-N you don#t use the dawn anymore due to TWR issues. So all in all, only a very tiny fraction of the playerbase would ever notice any change in it.

 

But in general, I completely agree with your assesment, it is very well thougt through and very well peresented.

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10 hours ago, Streetwind said:

So they made LF/Ox, LF-solo and Monoprop tanks all have the same mass ratios

Monoprop tanks do not have the same mass ratios as LF tanks, the Monoprop tanks all have different mass ratios, the best (2.5m) has 8.5:1, the worst (roundified) has only 4:1.

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A well thought out and presented bid for sanity, and consistency that makes the gameplay solid? Sign me up!

Streetwind you should totally put this on the bug tracker I've seen issues brought up like inconsistent landing gear mass that get fixed it's worth a shot.

Edited by passinglurker
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Pretty sure this isn't an oversight or bug, the mass ratios got an equalization pass a few versions back (i.e. all the xenon tanks have very similar mass ratios now). Few people complain about the mass ratios (and believe me, people *do* look at them) because it's understood to be the penalty for using such a high efficiency engine, IMO.

I'd note that in the chart even the worst case scenario has an "effective" Isp ~2.5x that of the next best vacuum engine and ~5x that of the best chemical engine. So it is still a stellar performer even by that measure, IMO it doesn't need further buffing.

 

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On the other hand, how often do you actually see people using ions? Not very, it seems to me. Compare that with how often you see Nervs, ie all the blooming time. Seems like a pretty strong clue the ion could use a buff (or the Nerv a nerf) to me.

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3 minutes ago, cantab said:

On the other hand, how often do you actually see people using ions? Not very, it seems to me. Compare that with how often you see Nervs, ie all the blooming time. Seems like a pretty strong clue the ion could use a buff (or the Nerv a nerf) to me.

Ions have very low thrust, so you only use them for very light probes. As soon as you put crew on your craft you're pretty much in no-ion territory.

I don't think ion engines are not used a lot because they're underbalanced. They're not used a lot because the scenarios where you use them are limited. And the burn times measured in hours of course. But that's a compromise for the unpractical reality of real ion engines that run for days or weeks at a time.

 

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24 minutes ago, cantab said:

On the other hand, how often do you actually see people using ions? Not very, it seems to me. Compare that with how often you see Nervs, ie all the blooming time. Seems like a pretty strong clue the ion could use a buff (or the Nerv a nerf) to me.

I think that's more an effect of the small size of the Kerbin system than anything else. It's really rare to need 10+km/s of dV in orbit, and LV-Ns don't have the hassle of trying to power it.

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1 hour ago, Red Iron Crown said:

Pretty sure this isn't an oversight or bug, the mass ratios got an equalization pass a few versions back (i.e. all the xenon tanks have very similar mass ratios now). Few people complain about the mass ratios (and believe me, people *do* look at them) because it's understood to be the penalty for using such a high efficiency engine, IMO.

I'd note that in the chart even the worst case scenario has an "effective" Isp ~2.5x that of the next best vacuum engine and ~5x that of the best chemical engine. So it is still a stellar performer even by that measure, IMO it doesn't need further buffing.

 

The mass ratios for the LFO tanks maybe sure, but monoprop still seems to be all over the place and by all over the place I mean anything short of identical to the same degree that two lfo tanks have identical mass ratios is unacceptable.

 

I'm tired of people making excuses for inconsistent and ill conceived part ballance I'd rather all my parts play by the same set of rules so that they can be easily compared rather than have people go "oh this one has 5m/s higher crash tolerance so thats why the mass ratio was cut in half" and "yes I see that one where they fit 200 units into a part the size of a round 8 with 3/4's the mass fraction its fine its cost ratio is much worse and it's at the end of the tech tree"

As for ion engine balance in my opinion they are broken and underpowered. Sure they perform well on paper but when you try to use them you find that light and inefficient lfo engines can do most of the same small jobs lighter, cheaper, and with less fuss until you get into the high DV range at which point a big stageing lfo probe and the lifter needed to launch it combined is still cheaper than a equivalent ion probe not to mention not having to worry about the sun and you can refill lfo via isru. and then if you ignore all that and actually stack up enough xenon to hit that high dv range your burn time is long and impractical on top of the impracticalities of needing sunlight. 
 

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@passinglurker I wasn't precise there, I meant the xenon tanks got an equalization pass when the 1.25m one was added. I agree that the monoprop tanks are a bit all over the place.

Ions aren't "broken", they just don't work they way you think they should (and that's an entirely valid viewpoint). Please save "broken" for things that actually are.

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Just now, Red Iron Crown said:

@passinglurker I wasn't precise there, I meant the xenon tanks got an equalization pass when the 1.25m one was added. I agree that the monoprop tanks are a bit all over the place.

Ions aren't "broken", they just don't work they way you think they should (and that's an entirely valid viewpoint). Please save "broken" for things that actually are.

How does not having a niche not constitute being "broken"? Not all bugs in this game are kraken-tastic glitches. It's not just that they don't work how I think they should it's that they don't work how anyone who has ever wanted to use them thinks they should, but most of the time proposed fixes are blown off because of reused arguments that basically amount to "It's a small niche part. no one will actually notice that it is actually useless" or "look at that isp you get see it's balanced because that is the only stat that matters"

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You say that as if no one uses them ever, or as if your opinion is universal. I see many ion craft posted here on the forums, and I've made and used some myself. They're not broken or useless.

I am not trying to prevent you from suggesting changes to them, far from it. 

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On 4/8/2016 at 3:07 AM, Streetwind said:

Therefore there's a hard limit of dV you can never exceed with any given engine, and it depends entirely on the tank's mass ratio

Well, that's true in a single stage.  But this is what drop tanks are for.  An ion-powered ship that has multiple xenon tanks, and drops them as it empties them, can get stupidly large dV with no problem.

On 4/8/2016 at 3:07 AM, Streetwind said:

To illustrate, I've done some math on a vessel that is comprised of 10 tons of dry mass, including a Dawn engine. I've added enough fuel to hit several different propellant fraction targets (percentage of vessel mass that is fuel). At each of those targets, I determined how much more dV and TWR the vessel would get if its tank mass ratio was 9:1 instead of 2.2727:1... and then converted that into how much less "effective Isp" and "effective thrust" the Dawn gets compared to a hypothetical LF/Ox engine with the same stats.

Two issues I see with this:

  • If I've got a ship with 10 tons of dry mass, that's quite a bit heavier than Dawn engines are good for.  They're intended for small, light craft.
  • It sounds like you're assuming a propellant fraction based on just having one big set of ion tanks that you lug around with you the whole time.  If I'm building a heavily-fueld ion ship like that, then I'm Doing It Wrong (TM).  Nobody would build an ion ship like that.  Because of the poor mass fraction of the xenon tanks, I ditch them as soon as they're empty; then I get far better performance than the table you've worked out.

I view the crappy mass fraction of xenon tanks as a feature, not a bug.  It's a compressed-gas tank, so it has to be heavy to have the necessary mechanical strength, as @Kerbart said.  And yes, that has implications for spacecraft design, to which the solution is the same as any other one in KSP:  I design around it.

2 hours ago, cantab said:

On the other hand, how often do you actually see people using ions? Not very, it seems to me. Compare that with how often you see Nervs, ie all the blooming time.

Aside from the reasons that @Kerbart and @Red Iron Crown point out, I think there's another big reason there:  patience.  Ion burns take forever, and many KSP players simply don't have the patience for that.  And since the fact that the burn takes forever is the whole point of the ion engine as a game feature, there's no way you can tweak the engine itself (or its tanks) to fix that problem.

I think the way to fix that problem is to make physics warp more robust.  It currently tops out at 4x.  Here's my suggestion:  Add 8x, 16x, and 32x modes on top of that, then do some game tweaking to prevent ships from destroying themselves under those conditions.  If I were coding it, here's what I would do.  First, you can't engage a physics warp >4 unless your TWR is below a certain limit.  If you're over the limit, the warp arrow is grayed out, and trying to engage it will just get you a little message "Cannot exceed X physics warp with current engines" or some such.  Second, I would cause all reaction wheels to scale down their torque whenever the physics warp is >4x.

It wouldn't be quite as simple as what I've just described, there would be some edge cases to take care of and it wouldn't be trivial.  But it seems to me that it ought to be doable, and it would make ion ships a whole lot more fun.

1 hour ago, passinglurker said:

As for ion engine balance in my opinion they are broken and underpowered.

I dunno, I use them fairly often myself and quite enjoy them.  :)  But then, my patience level may be freakishly high... I don't have a problem with sitting for 10-15 minutes on max physics warp, watching a probe and sipping tea.  They do a great job in a very specific niche, and I use them for that all the time.  I think that the main way in which they're "broken" is the really long burn time that makes the player sit around forever, and allowing higher physics warp would go a long way towards mitigating that.

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21 hours ago, Nich said:

I do wonder if 9:1 is too high as if someone was to make an ion ship that is 80% zenon the dv might be too disproportionate

That's why part of my suggestion consists of lowering the Dawn's Isp to compensate.

Admittedly, you could get higher mass ratios than before, but two things make that impractical. First, xenon is expensive; and second, why would you need to? There exists no destination inside the KSP solar system that requires that kind of dV. You can ultimately go everywhere on chemical engines, even the LV-N is technically unnecessary. As such, this is really a question of "is it a problem if an engine that already produces more dV than you could ever need gets a scenario where it produces even more?". You might as well allow the players to build such a gimmicky thing, because the only reason it would ever be built would be for RP purposes (building a ship to leave the solar system etc.)

 

20 hours ago, Kosmognome said:

But I don't know if there really is a problem to solve. You use Xenon for upper stages when they are small enough not to warrant using an LV-N, and if you use can use an LV-N you don#t use the dawn anymore due to TWR issues. So all in all, only a very tiny fraction of the playerbase would ever notice any change in it.

It's not a "problem", no; it certainly doesn't render the game unplayable or anything. It's a mere, let's say, aesthetic fix. The only reason I even went and suggested it is because of the extremely minimal effort of implementatation. It's a tiny issue with a fix costing a tiny amount of effort, and as such, I felt that it has a chance. I'm a perfectionist, and I care about KSP a lot, and I want it to feel as polished as it can be. :)

 

13 hours ago, passinglurker said:

Streetwind you should totally put this on the bug tracker I've seen issues brought up like inconsistent landing gear mass that get fixed it's worth a shot.

I don't think that's the purpose of the bugtracker, tbh. The current situation is not a bug, especially not a bug in the 1.1 prerelease (what the bugtracker should be focused on right now). It's just something that I feel should be handled differently from the way it is handled right now. Therefore the suggestions forum is the perfect place for it.

 

3 hours ago, cantab said:

Seems like a pretty strong clue the ion could use a buff (or the Nerv a nerf) to me.

 

4 hours ago, Red Iron Crown said:

I'd note that in the chart even the worst case scenario has an "effective" Isp ~2.5x that of the next best vacuum engine and ~5x that of the best chemical engine. So it is still a stellar performer even by that measure, IMO it doesn't need further buffing.

This isn't about buffing the Dawn, though. From a pure stat perspective, it's already one of the strongest engines in the game, likely the strongest. It's just that its statted for a niche for which the game generates almost no demand, and thus it seems weak in most applications (since those applications demand other engines). Also, the tanks are at fault for removing part of the reason for why you'd use it, namely the downsizing of the payload so that the launcher can be smaller and lighter. Unfortunately, the tanks are so bad that a good part of the savings are eaten up by dry mass.

This suggestion is primarily about maintaining the Dawn's current performance while making it more easily comparable to other engines, by means of removing an unnecessary complexity that only an utterly negligible part of the playerbase is even aware of. Believe me, we tried implementing it on two separate occasions in NFP and both times it was a complete failure. I should know, I was the one explaining the same thing over and over again on every new page of the thread for weeks on end :P (Perhaps a slight dramatization, but it sure felt that way.)

 

1 hour ago, Snark said:

Two issues I see with this:

  • If I've got a ship with 10 tons of dry mass, that's quite a bit heavier than Dawn engines are good for.  They're intended for small, light craft.
  • It sounds like you're assuming a propellant fraction based on just having one big set of ion tanks that you lug around with you the whole time.  If I'm building a heavily-fueld ion ship like that, then I'm Doing It Wrong (TM).  Nobody would build an ion ship like that.  Because of the poor mass fraction of the xenon tanks, I ditch them as soon as they're empty; then I get far better performance than the table you've worked out.

Apologies if I can't really see that as a valid counterargument. The example I drafted up exists only to produce numbers I can put into a table; it's not meant to represent any real ship ingame. It also just so happened that I already had an excel sheet for comparing mass ratios on a 10 ton example payload at hand. :P

However, if you went and designed an actual ship ingame, and ran the numbers on that, you would find that the math turns up the exact same results. That is because it doesn't matter how much payload/dead mass you actually have, or how much of that mass is dedicated to engines, or anything of the sort. The only thing that matters is how the xenon tankage makes the propellant fraction scale, versus how other tanks make it scale. You can discover this relationship on any given single-stage vessel.

I also disagree with the opinion that "they're intended for small, light craft". Engines can be clustered. I've seen people tug E-class asteroids into orbit with ion-powered craft, which is something other players use 500-ton behemoths for. But even if ion drives were used only on small craft, that still doesn't have anything to do with how the tanks are statted up.

 

1 hour ago, Snark said:

I view the crappy mass fraction of xenon tanks as a feature, not a bug.  It's a compressed-gas tank, so it has to be heavy to have the necessary mechanical strength, as @Kerbart said.  And yes, that has implications for spacecraft design, to which the solution is the same as any other one in KSP:  I design around it.

This, on the other hand, is more of an actual argument. And here it practically boils down to gameplay versus realism. As I already described in my opening post, yes, it is entirely realistic for some fuels to have different tank mass ratios than other fuels. It's just, as my personal experience with modmaking has repeatedly shown, an incredibly unintuitive concept to the average KSP player.

That personal experience has led me to believe that it would make KSP a better game if it abandoned this disparity for the same of consistent gameplay. From your response, on the other hand, it would appear that you value the perceived depth of gameplay provided by this disparity.

The problem with depth and complexity however is that they're by no means the same thing, yet can easily be confused for one another... especially by game designers. Extra Credits has a great double-episode feature on the topic, if you're interested. The TL;DR of it all is that it's easy to make games complex, but hard to extract depth from complexity. The best games are those that strive to minimize complexity while maximizing depth. If a game designer makes the decision to implement a layer of complexity, he or she should do it only because said complexity allows the game to extract a large amount of depth from it. In other words, if there's wide-reaching advantages that allow the game to better engage the player at every turn. A good example would be having orbital mechanics in KSP. As you may know, Harvester originally was doubtful whether he should do it at all, because it is an incredibly complex topic that players generally know nothing about beforehand, and which games consistently avoid like the plague. But KSP manages to take this layer of complexity and turn it into a depth of gameplay that has already lifted it into the annals of gaming. You could easily make the argument that orbital mechanics is the very thing that makes KSP what it is, and people would find it difficult to argue against it.

Unfortunately, having this mass ratio disparity for xenon tanks in KSP does not fall under that header. For starters, the vast majority of the playerbase is completely oblivious to it, which is a clear sign that it doesn't serve any tangible purpose. Then, the secondary effects - the requirement to fudge the Dawn's stats to make it perform the way it should compared to other engines. The elemination of a clear advantage of IRL electric propulsion (mass savings in payloads). The difficulty in implementing other uses of xenon through modding in a balanced, attractice fashion. I could go on.

True, it's ultimately a minor issue, as I've acknowledged further up in this post. But it's also a very minor fix. If implementing this would require actual coding work, I wouldn't even dream of bringing it up, 'cause there are many other things which can use that development time way more. But this just... feels like such a low-hanging fruit, you know? A tiny tweak that, in my opinion, makes KSP a more consistent, more polished experience that functions more the way that players expect it to function. :)

Edited by Streetwind
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1 hour ago, Snark said:

I think the way to fix that problem is to make physics warp more robust.  It currently tops out at 4×.  Here's my suggestion:  Add 8×, 16×, and 32× modes on top of that, then do some game tweaking to prevent ships from destroying themselves under those conditions.  If I were coding it, here's what I would do.  First, you can't engage a physics warp >4 unless your TWR is below a certain limit.  If you're over the limit, the warp arrow is grayed out, and trying to engage it will just get you a little message "Cannot exceed X physics warp with current engines" or some such.  Second, I would cause all reaction wheels to scale down their torque whenever the physics warp is >4×.

That's a good idea, although with current hardware (but why no build for the future?) of limited benefit, as it'd be likely that at higher physics warp the Physics Delta Time/Frame is hit. So yeah, you'll be running at 32× game time, but game time is running at 1/8 × real time, and you still end up waiting 15 minutes. Not that I mind either; there's floors to be vacuumed and bathrooms to be cleaned, dishwashers to be emptied and laundry to be folded. So it's not that big of a deal to wait. But: with hardware two years from now it will run a lot faster. Hopefully by then the dishwasher empties itself!

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Actually, there's already a mod that allows higher physics timewarp and implements a fix for the strange part compression happening in stock physics timewarp. You can go at 10x and higher, with the vesel as rigid as if it was not warping at all. And I'm not seeing any big performance hits when playing with it. So this is definitely something that Squad could also implement. (Even though it's off topic for this thread... The mod is just cool enough to be worth bringing up. :))

 

Edited by Streetwind
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3 hours ago, Streetwind said:

I don't think that's the purpose of the bugtracker, tbh. The current situation is not a bug, especially not a bug in the 1.1 prerelease (what the bugtracker should be focused on right now). It's just something that I feel should be handled differently from the way it is handled right now. Therefore the suggestions forum is the perfect place for it.

You really should take a closer look at the bug tracker then issues can be filed under a number of different categories not just bugs. This would be a perfect submission as a "feedback" issue imo

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The thing is... I've almost never seen anyone build a craft with 50% xenon payload fraction, and in those cases, they stage away empty xenon tanks rather than keeping them, so the mass penalty is much less than your chart.

50% payload fraction is 28,560 m/s in a single stage I rarely put more than 8km/s in any ion craft I use, much more commonly... I've got 3km or less in xenon.

8km/s is only 18% of the mass being xenon. 3km/s is only 7% of the mass being xenon. The TWR for any payload is jsut terrible. I only have them as the last stage, or for multiple relatively small maneuvers. I can't image trying to use up 28 km/s with an ion drive craft.... ughhhhh.

I agree that it should be pretty much just as easy to store Xenon as liquid oxygen

O2 Boiling point: 90.188 K

O2 density when liquid, at b.p. : 1.141 g/cm3

Xe boiling point: 165.051 K

Xenon density when liquid, at b.p. : 2.942 g/cm3

Xenon tanks require less insulation/refrigeration because the liquid becomes gass at a temperature nearly 65C/k higher, which should save mass. The same mass also fits in a smaller volume, meaning the tank is smaller which should save mass. The piping and such does not need to be corrosion resistant like with O2, saving mass.

Realistically, Xenon tanks should be lighter than equivalent liquid oxygen tanks... but relative to completely non cryogenic tanks like hydrazine/kerosene/some non cryogenic oxidizers, I don't know. It would be a heck of a lot better than LH2 tanks.

In terms of gameplay, with the dV one needs in KSPs small system, and the Isp of 4,200, the mass fraction that needs to be Xenon is so low that the tank mass doesn't change things much.

I support giving them better mass ratios, but I don't think it actually changes much

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  • 3 months later...

I think the stock High Mass ratio nerve is mend to blanced the Super High Thrust of the Ion engine, which in realsiticly should be 2000 times weaker! From an conservation of energy perspective, the Ion engine is current equivalent to super efficient fusion reactor which manages to fuse Xenon atoms with a Q factor of 2000 and directly convert in into propulsion with 100% efficiency! So lowering the mass ratio of the Xenon tanks is extremly generous from the developers. If you want to improve the mass fraction of the Xenon tanks, you should also lower the thrust of the Ion engine, otherwise they would disproportionatly become extremely overpowered.

Edited by FreeThinker
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55 minutes ago, FreeThinker said:

I think the stock High Mass ratio nerve is mend to blanced the Super High Thrust of the Ion engine, which in realsiticly should be 2000 times weaker! From an conservation of energy perspective, the Ion engine is current equivalent to super efficient fusion reactor which manages to fuse Xenon atoms with a Q factor of 2000 and directly convert in into propulsion with 100% efficiency! So lowering the mass ratio of the Xenon tanks is extremly generous from the developers. If you want to improve the mass fraction of the Xenon tanks, you should also lower the thrust of the Ion engine, otherwise they would disproportionatly become extremely overpowered.

Which is precisely what I proposed in the OP, four months ago...

:P

FYI: this stuff is earmarked by Squad for the rocketry part revamp, last I heard. Implementation was actually tested during the 1.1 experimentals, a day or two after I made this suggestion, but it ended up breaking a tutorial (because they also wanted to normalize the mass of RCS tanks as well --> things got complicated fast). So it got shifted back to 1.2. Or whenever the rocketry part revamp is going to happen.

You'll probably see several other such tweaks being made at that time.

Edited by Streetwind
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12 minutes ago, Streetwind said:

Which is precisely what I proposed in the OP, four months ago...

:P

One thing which isn't mentioned but should be taken into consideration are any cryogenic/cooling requirements. My guess is that the 9:1 mass ratio of the Dawn Xenon storage can only be achieved if the Xenon is in a liquid state, which would mean storage would require continous cooling. If not, do you know any document proving the 9:1 mass ratio was for compressed Xenon Tank?

Edited by FreeThinker
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...Did you read any part of the opening post at all, beyond the title? :s This is not, and never was at any point, about improving realism. I think I made it quite clear why I think that that number should be selected. What mass ratios real life xenon tanks can achieve has had no bearing on it.

Though for what it's worth: Kerbas_ad_astra, author of SMURFF, estimates (normal, non-cryogenic) xenon tanks at about 90% fuel by mass (that's a ratio of 10:1), citing he picked the median after seeing sources for anywhere from 85% to 95%. I'm sure that if you ask him, he can point you at where he saw it.

 

Edited by Streetwind
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55 minutes ago, Streetwind said:

This is not, and never was at any point, about improving realism. I think I made it quite clear why I think that that number should be selected. What mass ratios real life xenon tanks can achieve has had no bearing on it.

Realy, nothing at all?

On 8-4-2016 at 0:07 PM, Streetwind said:

Why It Should Be Addressed:

In real life, the mass ratio of a fuel tank depends on a great many factors, including the physical properties of the propellant itself. So you could argument that it is realistic that different fuels have tanks with different mass ratios. However, for xenon specifically, real life tanks are not anywhere near that low.

 

alright , your main goal is not realism, but simplicity. But I think this is flauwed Idea because hidden facts like lower mass fraction for different fuels/propellants are what makes KSP intresting and educational. I say let players come to the realisation their vessel isn't as effective as they though it was (which I did btw), that will teach them that High Isp isn't everything, just like the countless times they failed launching their rocket. KSP is all about learning by failure.

Edited by FreeThinker
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